It is going to be difficult to operate a hovercraft wth two channels only, really need three for thrust , rudder and lift fan but you could use a Action Quadswitch to swtch the lift fan on and off by pulsing the rudder stick. This would be better than a micro switch on the rudder, tendency is to switch off when turning if not very careful. The lift fan motor would be direct feed from battery possibly via a relay if the amperage is too high for the switch. If you need to throttle the lift motor , try Maplins voltage regulator RN41U (£14.99)